1. Field of the Invention
The present invention relates to an image capturing apparatus and a control method therefor and, more specifically, to an image capturing apparatus that performs automatic focus detection and a control method therefor.
2. Description of the Related Art
Conventionally, a focus detection apparatus in a camera which receives light that has passed through an imaging lens and accumulates charge by an AF sensor, and detects a defocus amount on the basis of an image signal of a subject read out from the AF sensor is known. Meanwhile, there is a control method, adapted in focus detection apparatuses, to keep focusing on a moving subject by driving an imaging lens in connection with the movement of the subject. In order to keep focusing on a moving subject, focus detection is generally performed for each frame, and an imaging lens is moved in accordance with a value obtained by adding an amount to a predicted future position of the moving subject to the focus detection result. In predicting the future position of the moving subject, an amount (correction amount) that the subject will move during a period since mirror-up operation is started after the focus detection is ended until an image sensor starts to be exposed as shown in FIG. 4A. Conventionally, many focus control apparatuses having a variety of means for obtaining such correction amount have been proposed.
For example, Japanese Patent No. 3,445,311 discloses means that calculates a correction amount on the basis of the sum of a mirror-up time, that has measured at a previous time, and a driving time corresponding to a time lag correction amount calculated on the basis of a following speed that is set in accordance with a moving speed of a subject. Further, Japanese Patent No. 4,689,023 discloses a focus control apparatus that sets the number of times focus detection is to be performed in accordance with a set continuous shooting rate, controls to detect a focus amount and move an imaging lens for the set number of times during each interval between shootings in the continuous shooting.
Meanwhile, there is an image shooting mode (silent image shooting mode) that enables silent capturing of an image by performing the mirror-up and mirror-down operation more quietly (i.e., moving the mirror slowly) than in a normal image shooting mode. In the silent image shooting mode, as shown in FIG. 4B, it is possible to perform image capturing operation quietly by taking a longer time to move up and down the mirror than in the normal image shooting mode. Further, there is a camera that requires time since mirror-up operation is started until a mirror actually starts to move up as shown in FIG. 4B due to a mechanical structure. For example, a conventional camera having such mechanical structure that controls driving of a main mirror and a focal plane shutter using one motor requires a charging period of the motor in order to perform mirror-up operation slowly. This charging period is the main cause of taking time since the mirror-up operation is started until the mirror actually starts to move up. In cameras which take time for mirror-up operation and cameras which take time before a mirror actually starts to move up, a period of time since focus detection is performed last time until exposure starts is long, and thus an interval (prediction time gap) between a time at which a moving amount of a subject is predicted and a time at which the subject is actually shot becomes long.
In an example of an apparatus disclosed in Japanese Patent No. 3,445,311, if a prediction time gap becomes longer, it becomes more difficult to predict how the subject moves during the interval. If the accuracy of the predicted moving amount of the subject deteriorates, a possibility that the subject will be properly focused decreases accordingly. In an example of an apparatus disclosed in Japanese Patent No. 4,689,023, it is considered that the time taken to move up and down a mirror becomes long, which causes deterioration of a predicted moving amount of the subject, and a possibility that the subject will be properly focused decreases as in the case of Japanese Patent No. 3,445,311.